As integrated circuits (ICs) have become finer and more sophisticated, projection exposure apparatus to transfer circuit patterns onto semiconductor wafers or other receptive substrates (e.g. glass plates, etc.) have been required to perform at higher and higher resolutions. Particularly in recent years, systems have been required to perform well not only in an ideal environment but to do so even when the surrounding conditions change.
Although current projection exposure systems perform satisfactorily in an ideal environment, such systems typically do not do as well when conditions such as atmospheric pressure and temperature surrounding the projection optical system in the projection exposure system change. In conventional projection exposure systems, magnification errors due to changes in atmospheric pressure are corrected by controlling the air pressure of the medium which fills the gaps between the multiple optical members comprising the projection optical system. However, this corrective measure merely corrects for magnification abnormalities. The deterioration of other imaging performances is merely reduced to an amount where it is not considered a problem in a practical application of the device, with the practical application of the device being defined by the performance guaranteed atmospheric pressure range (the range for which system's operating performance is guaranteed). As for changes caused by deviations in temperature, various temperature controlling devices are known which are used to control the temperature of projection optical system. These devices essentially maintain the temperature of the system at sufficiently close to that of an ideal environment, so the performance required from such conventional systems can be obtained.
However, in recent years, such methods to correct for atmospheric pressure changes have become unsuitable for the needs of the users of such exposure systems who are trying to produce the aforementioned finer and more complicated circuit patterns. The greater densities being called for by semiconductor manufacturers are requiring higher resolutions from such projection exposure systems. This, in turn, requires a narrowing in the performance guaranteed atmospheric pressure range to sufficiently satisfy the rigorous performance being called for by the users of these projection exposure systems. Further, recent calls for higher resolution and more rigorous performance from such systems have also resulted in a need for stricter temperature control in such systems.
Differences in atmospheric pressure are inevitable due to changes in the altitude, etc. This is due in part to the fact that the manufacturing site of a projection optical system and the location where the projection exposure apparatus into which the optical system is incorporated are not the same. Conventionally, a projection optical system is adjusted until it achieves ideal imaging performance at the manufacturing site. The optical system is then further adjusted at the location of the actual use according to atmospheric conditions, e.g., altitude, in order to obtain the same ideal imaging performance at the use site. This adjustment is typically accomplished by, for example, changing the multiple air gaps within the projection optical system, or exchanging one lens element for another having a slightly different curvature radius, and so forth. Therefore, when the place where the projection optical system is used changes (e.g. different altitude), a significant readjustment of the projection optical system is necessary, resulting in disassembly and re-assembly of the projection optical system. As for temperature control, it is necessary to set the environmental temperature for each actual use site before manufacturing with the projection optical system.
Accordingly, there is a need for a projection optical system which is capable of maintaining the performance of an ideal environment, even when the actual use environment differs substantially from the ideal environment. Further, there is a need for a projection optical system which is capable of adapting to the environment in which it is used with relatively minor adjustments to the projection optical system.